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screen.asm
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screen.asm
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;vars
screen_buffer_nbr !byte 0
screen_base !word 0 ; plus 1 row
screen_back_buffer_base !word 0
raster_1 !byte 0
raster_2 !byte 0
ROWS_PER_COPY = 12
; --------- screen_clear was taken from https://gist.github.com/actraiser/8e335033b2622409bd96 -----
screen_clear ldx #$00 ; set X to zero (black color code)
stx $d021 ; set background color
stx $d020 ; set border color
+set16im $0400, screen_base ; init screen pointers
+set16im $0800, screen_back_buffer_base
_screen_clear_loop
lda #$20 ; #$20 is a blank char
sta $0400,x ; fill four areas with 256 spacebar characters
sta $0500,x
sta $0600,x
sta $06e8,x
lda #$00 ; set foreground to black in Color Ram
sta $d800,x
sta $d900,x
sta $da00,x
sta $dae8,x
inx
bne _screen_clear_loop ; did X overflow to zero yet?
rts ; return from this subroutine
screen_init
lda $d016 ; d016 is VIC-II control register.
and #%11110111 ; un-set bit 3 to enable 38 column mode
sta $d016
rts
screen_swap lda screen_buffer_nbr ; toggle screen_buffer_number between 0 and 1
eor #$01
sta screen_buffer_nbr
bne screen_swap_to_1
;set screen ptr to screen 0
lda $d018 ; top 4 bits of d018 holds the screen location in RAM
and #$0f ; mask upper 4 bits
ora #$10 ; set upper 4 bits to '1'
sta $d018
+set16im $0400, screen_base
+set16im $0800, screen_back_buffer_base
rts
screen_swap_to_1 ;set screen ptr to screen 1
lda $d018
and #$0f
ora #$20
sta $d018
+set16im $0800, screen_base
+set16im $0400, screen_back_buffer_base
rts
screen_shift_lower ; copy the lower half of screen to back buffer
+set16 screen_base, screen_ptr
inc screen_ptr ; shift columns over by 1 while copying
+set16 screen_back_buffer_base, screen_ptr_dest
+add16im screen_ptr, 40*ROWS_PER_COPY, screen_ptr
+add16im screen_ptr_dest, 40*ROWS_PER_COPY, screen_ptr_dest
ldy #0 ; y is the current column
ldx #ROWS_PER_COPY ; x is the nbr of rows to copy
jmp video_ram_copy_line
screen_shift_upper
+set16 screen_base, screen_ptr
inc screen_ptr ; shift columns over by 1 at the same as copying to back buffer
+set16 screen_back_buffer_base, screen_ptr_dest
ldy #0 ; y is the current column
ldx #ROWS_PER_COPY ; x is the nbr of rows to copy
video_ram_copy_line ; copy a line of screen data to back buffer
; we unroll the loop for better performance
lda (screen_ptr), y
sta (screen_ptr_dest), y
iny
lda (screen_ptr), y
sta (screen_ptr_dest), y
iny
lda (screen_ptr), y
sta (screen_ptr_dest), y
iny
lda (screen_ptr), y
sta (screen_ptr_dest), y
iny
lda (screen_ptr), y
sta (screen_ptr_dest), y
iny
lda (screen_ptr), y
sta (screen_ptr_dest), y
iny
lda (screen_ptr), y
sta (screen_ptr_dest), y
iny
lda (screen_ptr), y
sta (screen_ptr_dest), y
iny
lda (screen_ptr), y
sta (screen_ptr_dest), y
iny
cpy #36
bne video_ram_copy_line
; now copy the last 3 bytes
lda (screen_ptr), y
sta (screen_ptr_dest), y
iny
lda (screen_ptr), y
sta (screen_ptr_dest), y
iny
lda (screen_ptr), y
sta (screen_ptr_dest), y
dex ; have we copied all the rows?
beq video_ram_copy_done
ldy #0 ; reset column number
+add16im screen_ptr, 40, screen_ptr
+add16im screen_ptr_dest, 40, screen_ptr_dest
jmp video_ram_copy_line ; copy another line
video_ram_copy_done
rts
color_shift_upper ; copy color RAM. Uses same code as screen_shift
+set16im $d801, screen_ptr
+set16im $d800, screen_ptr_dest
ldy #0 ; y is the current column
ldx #ROWS_PER_COPY ; nbr of rows to copy
jmp video_ram_copy_line
color_shift_lower
+set16im $d801+40*ROWS_PER_COPY, screen_ptr
+set16im $d800+40*ROWS_PER_COPY, screen_ptr_dest
ldy #0 ; y is the current column
ldx #ROWS_PER_COPY ; nbr of rows to copy
jmp video_ram_copy_line